3.2.5 Transition metals Flashcards
Define transition metals
A metal that can form one or more stable ions with a partially filled d sub-level
Which two period 4 d-block elements aren’t transition metals, why?
Scandium, as it only forms one ion, Sc3+, which has an empty d sub-level.
Zinc, as it only forms one ion, Zn2+, which has a full d sub-level.
Which electrons are removed first when forming a positive transition metal ions?
Electrons from the s block, the electrons from the d block
What are transition metals similar physical properties?
- They all have a high density
- They all have high melting and boiling points
- Their ionic radii are approximately the same
What chemical properties do transition metals possess?
- They form complex ions
- They form coloured ions
- They’re good catalysts
- They can exist in variable oxidation states
What is a complex?
A central metal atom or ion surrounded by co-ordinately bonded ligands
What is a ligand?
An atom or ion that donates a pair of electrons to a central transition metal ion to form a co-ordinate bond
What is co-ordination number?
The number of co-ordinate bonds that are formed with the central metal ion
What are the normal co-ordination numbers, why is this?
6, 4, and 2
If ligands are small (e.g. H2O or NH3) 6 can fit around the central metal ion. But if ligands are larger (e.g. Cl-) only 4 can fit around the central metal ion
What shape does co-ordinate number of 6 produce?
What is the bond angle?
Octahedral, bond angles are all 90o
What shape does a co-ordinate number of 4 produce?
What is the bond angle?
Tetrahedral, bond angles are all 109.5o, however, can form a square planar shape, where bond angles are 90o
What shape does a co-ordinate number of 2 produce?
What is the bond angle?
Linear shape, bond angle 180o
What is the equation to work out the oxidation state of the metal ion
Oxidation state of the metal ion = Total oxidation state - Sum of the oxidation states of the ligands
What is a monodentate ligand?
A ligand that can only form one co-ordinate bond
What is a bidentate ligand?
A ligand that can can form two co-ordinate bonds
What is a multidentate ligand
A ligand able to form two or more co-ordinate bonds
Explain the structure of the haem group in haemoglobin?
- Haemoglobin contains Fe2+ ions, which are hexa-coordinated (six lone pair are donatated to them to form six co-ordinate bonds in an octohedral structure
- Four of the co-ordinate bonds come from a single multidentate ligand. Four nitrogen atoms from the same molecule co-ordinate bond around the Fe2+ to form a circle, aka haem
- The other two co-ordinate bonds come from a portien called globin, and either an oxygen or a water molecule - so the complex can transport oxygen to where it’s needed, and then swap it for a water molecule
How does haemoglobin work?
- In the lung, where conc. of oxygen is high, an oxygen molecule substitutes the water ligand and bonds co-ordinately to the Fe(II) ion to form oxyhaemoglobin
- Oxyhaemoglobin is carried around the body
- When oxyhaemoglobon gets to a place where oxygen is needed, the oxygen molecule is exchanged for a water molecule
- The haemoglobin the return to the lungs to start the process again
How does inhaling carbon monoxide disrupt haemoglobins ability to transport O2?
- The haemoglobin swaps substitutes its water ligand for a carbon monoxide ligand, forming carboxyhaemoglobin
- Carbon monoxide is a strong ligand and doesn’t readily exchange with oxygen or water ligands
- Therefore haemoglobin cannot transport oxygen any more
What are the symptoms of carbon monoxide poisoning?
- Headaches
- Dizziness
- Unconsciousness
- Death
How can complex ions show optical isomerism?
- Complex ions show optical isomerism when an ion can exist in two forms that are non-superimposable mirror images
- This happens with octohedral complexes when three bidentate ligands co-ordinately bond with a central metal ion
Explain how cis-trans isomers can be form in octohedral complex ions
Octohedral complexes with four monodentate ligands of one type and two monodentate ligands of another type can show cis-trans isomerism. If the odd ligands are opposite, it’s the trans isomer. If the they are next to each other it’s the cis isomer
Explain how cis-trans isomers can be form in square planar complex ions
Square planar complex ions that have two pairs of ligands show cis-trans isomerism. When the paired ligands are opposite each other it’s the trans isomer, when they are next to eachother it’s the cis isomer
E.g. cis/transplatin
Explain how ligands split the 3d sub-level into two energy levels
Normally the 3d orbitals of a transition metal all have the same energy, when a ligand bonds to the transition metal ion, some of the orbitals gain energy, splitting the 3d sub-level in two different energy levels
1. Electons tend to occupy the lower orbitals (ground state), to be promoted to the higher level they need equal energy to the energy gap, ΔE.
2. This energy come from visible light
3. The amount of energy needed to promote electrons depends on the central metal ion and it’s oxidation state, the ligands and the co-ordination number, as these effect the size of the energy gap, ΔE.
Give the formula that can be used to determine the energy absorbed when electrons are promoted from the ground state to the excited state
ΔE=hv=hc/λ
- v= frequency of the light absorbed (Hz)
- h= plank’s constant (6.63x10-34J s)
- c= the speed of light (3.00x108 m s-1)
- λ= wavelength of light absorbed (m)
Why do complex ions appear coloured?
- When visible light hits a transition metal, some frequencies are absorbed when electrons are promoted to higher orbitals, the frequency absorbed depends of the energy gap (ΔE) - the larger the energy gap the higher the frequency absorbed
- The rest of the frequencies are transmitted or reflected and combine to produce complementary colour
- If there are no 3d electrons, or the 3d sub-level is full then no electrons will be promoted so no energy will be absorbed. Therefore the solution will appear white or colourless
Name factors that effect the colour of complex ions (effect the energy gap)
- Changes in oxidation state
- Changes in co-ordination number
- Changes in ligand
Give an two examples of how changes in oxidation state can change the colour of a complex ion
Complex: [Fe(H2O)6]2+(aq) -> [Fe(H2O)6]3+(aq)
Ox. state: 2+ -> 3+
Colour: Pale green -> Purple
AND…
Complex: [V(H2O)6]2+(aq) -> [V(H2O)6]3+(aq)
Ox state: +2 -> +3
Colour: violet -> green
Give an example of how co-ordination number changes the colour of a complex ion
Complex: [Cu(H2O)6]2+ + 4Cl- -> [CuCl4]2- + 6H2O
Co-ordination number: 6 -> 4
Colour: blue -> yellow
Give an example of how changing the ligand can effect the colour of a complex ion despite co-ordination number and oxidation state staying the same
[Co(H2O)6]2+ + 6NH3 -> [Co(NH3)6]2+ + 6H2O
Oxidation state: +2 -> +2
Colour: pink -> straw coloured
Explain how optical spectroscopy can be used to determine the concentration of a solution of complex ions
- White light is shone through a filter, which is chosen to only let the colour of light that is absorbed through the sample
- The light passes through the sample to a colorimeter, which calculates how much light was absorbed by the sample
- The more concentrated a coloured solution is the more light it will absorb. So you can use this measurement to work out the conc. of a solution of transition metal ions
What needs to be done before using optical spectrocopy to calculate the conc. of an unknown solution?
A calibration curve needs to be produced, to produce a calibration curve a substance of a known conc. is measured and the results are plotted on a graph. One this has been done the graph can be used to determine the conc. of an unknown substance
Give an example of a complete ligand substitution, were the ligands are of a similar size and are the same charge
The co-ordination number and shape of the complex ion won’t change
[Co(H2O)6]2+(aq) + 6NH3(aq) -> [Co(NH3)6]2+(aq) + 6H2O(aq)
Colour goes from pink to straw coloured
Give an example of a partial ligand substitution, were the ligands are of a similar size and are the same charge
The co-ordination number and shape of the complex ion won’t change
[Cu(H2O)6]2+(aq) + 4NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)
Colour goes from blue to deep blue
Ammonia in excess*